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FreeBSD-SA-06:09.openssh                                    Security Advisory
                                                          The FreeBSD Project

Topic:          Remote denial of service in OpenSSH

Category:       contrib
Module:         OpenSSH
Announced:      2006-03-01
Affects:        FreeBSD 5.3 and 5.4
Corrected:      2006-03-01 14:19:48 UTC (RELENG_5, 5.5-PRERELEASE)
                2006-03-01 14:21:01 UTC (RELENG_5_4, 5.4-RELEASE-p12)
                2006-03-01 14:24:52 UTC (RELENG_5_3, 5.3-RELEASE-p27)
CVE Name:       CVE-2006-0883

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit
<URL:http://www.freebsd.org/security/>.

0.   Revision History

v1.0 2006-03-01  Initial release.
v1.1 2006-03-01  Corrected workaround instructions.
v1.2 2006-03-03  Further correction to workaround.

I.   Background

OpenSSH is an implementation of the SSH protocol suite, providing an
encrypted, authenticated transport for a variety of services,
including remote shell access.

Privilege separation is a mechanism used by OpenSSH to protect itself
against possible future vulnerabilities.  It works by splitting the
server process in two: the child process drops its privileges and
carries on the conversation with the client, while the parent retains
its privileges, monitors the child, and performs privileged operations
on behalf of the child when it is satisified that everything is in
order.  Privilege separation is enabled by default in FreeBSD.

OpenPAM is an implementation of the PAM framework, which allows the
use of loadable modules to implement user authentication and session
management in a manner defined by the administrator.  It is used by
OpenSSH and numerous other applications in FreeBSD to provide a
consistent and configurable authentication system.

II.  Problem Description

Because OpenSSH and OpenPAM have conflicting designs (one is event-
driven while the other is callback-driven), it is necessary for
OpenSSH to fork a child process to handle calls to the PAM framework.
However, if the unprivileged child terminates while PAM authentication
is under way, the parent process incorrectly believes that the PAM
child also terminated.  The parent process then terminates, and the
PAM child is left behind.

Due to the way OpenSSH performs internal accounting, these orphaned
PAM children are counted as pending connections by the master OpenSSH
server process.  Once a certain number of orphans has accumulated, the
master decides that it is overloaded and stops accepting client
connections.

III. Impact

By repeatedly connecting to a vulnerable server, waiting for a
password prompt, and closing the connection, an attacker can cause
OpenSSH to stop accepting client connections until the system restarts
or an administrator manually kills the orphaned PAM processes.

IV.  Workaround

The following command will show a list of orphaned PAM processes:

# pgrep -lf 'sshd.*\[pam\]'

The following command will kill orphaned PAM processes:

# pkill -f 'sshd.*\[pam\]'

To prevent OpenSSH from leaving orphaned PAM processes behind, perform
one of the following:

1) Disable PAM authentication in OpenSSH.  Users will still be able to
   log in using their Unix password, OPIE or SSH keys.

   To do this, execute the following commands as root:

# echo 'UsePAM no' >>/etc/ssh/sshd_config
# echo 'PasswordAuthentication yes' >>/etc/ssh/sshd_config
# /etc/rc.d/sshd restart

2) If disabling PAM is not an option - if, for instance, you use
   RADIUS authentication, or store user passwords in an SQL database -
   you may instead disable privilege separation.  However, this may
   leave OpenSSH vulnerable to hitherto unknown bugs, and should be
   considered a last resort.

   To do this, execute the following commands as root:

# echo 'UsePrivilegeSeparation no' >>/etc/ssh/sshd_config
# /etc/rc.d/sshd restart

V.   Solution

Perform one of the following:

1) Upgrade your vulnerable system to 5-STABLE or to the RELENG_5_4 or
   RELENG_5_3 security branch dated after the correction date.

2) To patch your present system:

The following patches have been verified to apply to FreeBSD 5.3 and
5.4 systems.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-06:09/openssh.patch
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-06:09/openssh.patch.asc

b) Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch
# cd /usr/src/secure/lib/libssh
# make obj && make depend && make && make install
# cd /usr/src/secure/usr.sbin/sshd
# make obj && make depend && make && make install
# /etc/rc.d/sshd restart

VI.  Correction details

The following list contains the revision numbers of each file that was
corrected in FreeBSD.

Branch                                                           Revision
  Path
- -------------------------------------------------------------------------
RELENG_5
  src/crypto/openssh/auth-pam.c                                  1.14.2.1
  src/crypto/openssh/ssh_config                                  1.25.2.1
  src/crypto/openssh/ssh_config.5                                1.15.2.1
  src/crypto/openssh/sshd_config                                 1.40.2.1
  src/crypto/openssh/sshd_config.5                               1.21.2.1
  src/crypto/openssh/version.h                                   1.27.2.1
RELENG_5_4
  src/UPDATING                                            1.342.2.24.2.21
  src/sys/conf/newvers.sh                                  1.62.2.18.2.17
  src/crypto/openssh/auth-pam.c                                  1.14.6.1
  src/crypto/openssh/ssh_config                                  1.25.6.1
  src/crypto/openssh/ssh_config.5                                1.15.6.1
  src/crypto/openssh/sshd_config                                 1.40.6.1
  src/crypto/openssh/sshd_config.5                               1.21.6.1
  src/crypto/openssh/version.h                                   1.27.6.1
RELENG_5_3
  src/UPDATING                                            1.342.2.13.2.30
  src/sys/conf/newvers.sh                                  1.62.2.15.2.32
  src/crypto/openssh/auth-pam.c                                  1.14.4.1
  src/crypto/openssh/ssh_config                                  1.25.4.1
  src/crypto/openssh/ssh_config.5                                1.15.4.1
  src/crypto/openssh/sshd_config                                 1.40.4.1
  src/crypto/openssh/sshd_config.5                               1.21.4.1
  src/crypto/openssh/version.h                                   1.27.4.1
- -------------------------------------------------------------------------

VII. References

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=248125
http://bugzilla.mindrot.org/show_bug.cgi?id=839
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2006-0883

The latest revision of this advisory is available at
ftp://ftp.freebsd.org/pub/FreeBSD/CERT/advisories/FreeBSD-SA-06:09.openssh.asc
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